The present invention relates to a solar energy absorber for use with a linear optical concentrating system, that is, an optical system which focuses radiant energy at a focal axis, and to solar energy absorption apparatus incorporating such an absorber combined with such an optical concentrating system.
Solar energy absorbers associated with linear optical concentrating systems generally consist of a cylindrical tube disposed along the focal axis of the optical concentrating system and carrying a fluid flow which takes up energy absorbed by the tube in the form of heat. The outer surface of the cylindrical tube is usually covered with a black film to improve the absorbing characteristics of the tube and to reduce infra-red emmission and the tube is generally housed within a glass casing to limit heat loss by radiation. A space is also left between the cylindrical tube and the glass casing in which a steady vacuum is formed to limit heat losses by convection and conduction.
Solar energy absorbers of the cylindrical-tube type described above have various disadvantages and, more particularly:
(a) heat losses by conduction, convection and radiation increase with increase in temperature of the absorbing tube and the heat exchange conditions between the tube and the fluid passing therethrough deteriorate along the tube in the direction of flow. Thus the heat yield to the fluid decreases along the tube in the direction of the fluid flow therethrough. PA1 (b) the methods used in making this type of solar energy absorber are becoming more and more difficult and costly. PA1 (c) the black films currently in use will not withstand temperatures greater than 300.degree. C. Their properties deteriorate after a given number of thermal cycles resulting in particularly burdensome problems of maintenance. Furthermore, it is very difficult to achieve uniform deposition of the film in the first instance. PA1 (d) infra-red radiation emitted by the cylindrical tube is absorbed by the glass casing and results in heat loss by convection since the glass casing is in direct contact with the surrounding air. The use of dichroic deposits on the glass casing to reduce radiation reflection in the visible range and to increase it in the infra-red range is being studied, but here one is dealing with techniques which are difficult, costly and usable in practice only for small casings. PA1 (e) given the usual dimensions of the cylindrical absorbing tube, it is extremely difficult to achieve a steady vacuum between the cylindrical tube and the glass casing which will limit the heat loss effectively.